The present invention is generally directed to a surgical treatment for glaucoma, and relates more particularly to a device and method for continuously maintaining the patency of Schlemm""s canal with a trough-like, indwelling stent which can be surgically placed to traverse at least a portion of the circumference of the canal and to facilitate the drainage of aqueous humor therethrough.
Glaucoma is a significant public health problem, because glaucoma is a major cause of blindness. The blindness that results from glaucoma involves both central and peripheral vision and has a major impact on an individual""s ability to lead an independent life.
Glaucoma is an optic neuropathy (a disorder of the optic nerve) that usually occurs in the setting of an elevated intraocular pressure. The pressure within the eye increases and this is associated with changes in the appearance (xe2x80x9ccuppingxe2x80x9d) and function (xe2x80x9cblind spotsxe2x80x9d in the visual field) of the optic nerve. If the pressure remains high enough for a long enough period of time, total vision loss occurs. High pressure develops in an eye because of an internal fluid imbalance.
The eye is a hollow structure that contains a clear fluid called xe2x80x9caqueous humor.xe2x80x9d Aqueous humor is formed in the posterior chamber of the eye by the ciliary body at a rate of about 2.5 microliters per minute. The fluid, which is made at a fairly constant rate, then passes around the lens, through the pupillary opening in the iris and into the anterior chamber of the eye. Once in the anterior chamber, the fluid drains out of the eye through two different routes. In the xe2x80x9cuveoscleralxe2x80x9d route, the fluid percolates between muscle fibers of the ciliary body. This route accounts for approximately ten percent of the aqueous outflow in humans. The primary pathway for aqueous outflow in humans is through the xe2x80x9ccanalicularxe2x80x9d route that involves the trabecular meshwork and Schlemm""s canal.
The trabecular meshwork and Schlemm""s canal are located at the junction between the iris and the sclera. This junction or corner is called xe2x80x9cthe angle.xe2x80x9d The trabecular meshwork is a wedge-shaped structure that runs around the circumference of the eye. It is composed of collagen beams arranged in a three-dimensional sieve-like structure. The beams are lined with a monolayer of cells called trabecular cells. The spaces between the collagen beams are filled with an extracellular substance that is produced by the trabecular cells. These cells also produce enzymes that degrade the extracellular material. Schlemm""s canal is adjacent to the trabecular meshwork. The outer wall of the trabecular meshwork coincides with the inner wall of Schlemm""s canal. Schlemm""s canal is a tube-like structure that runs around the circumference of the cornea. In human adults, Schlemm""s Canal is believed to be divided by septa into a series of autonomous, dead-end canals.
The aqueous fluid travels through the spaces between the trabecular beams, across the inner wall of Schlemm""s canal into the canal, through a series of about 25 collecting channels that drain from Schlemm""s canal and into the episcleral venous system. In a normal situation, aqueous production is equal to aqueous outflow and intraocular pressure remains fairly constant in the 15 to 21 mmHg range. In glaucoma, the resistance through the canalicular outflow system is abnormally high.
In primary open angle glaucoma, which is the most common form of glaucoma, the abnormal resistance is believed to be along the outer aspect of trabecular meshwork and the inner wall of Schlemm""s canal. It is believed that an abnormal metabolism of the trabecular cells leads to an excessive build up of extracellular materials or a build up of abnormally xe2x80x9cstiffxe2x80x9d materials in this area. Histopathology of glaucoma eyes also demonstrates a collapse of Schlemm""s canal. Primary open angle glaucoma accounts for approximately eighty-five percent of all glaucoma. Other forms of glaucoma (such as angle closure glaucoma and secondary glaucomas) also involve decreased outflow through the canalicular pathway but the increased resistance is from other causes such as mechanical blockage, inflammatory debris, cellular blockage, etc.
With the increased resistance, the aqueous fluid builds up because it cannot exit fast enough. As the fluid builds up, the intraocular pressure (IOP) within the eye increases. The increased IOP compresses the axons in the optic nerve and also may compromise the vascular supply to the optic nerve. The optic nerve carries vision from the eye to the brain. Some optic nerves seem more susceptible to IOP than other eyes. While research is investigating ways to protect the nerve from an elevated pressure, the only therapeutic approach currently available in glaucoma is to reduce the intraocular pressure.
The clinical treatment of glaucoma is approached in a step-wise fashion. Medication often is the first treatment option. Administered either topically or orally, these medications work to either reduce aqueous production or they act to increase outflow. Currently available medications have many serious side effects including: congestive heart failure, respiratory distress, hypertension, depression, renal stones, aplastic anemia, sexual dysfunction and death. Compliance with medication is a major problem, with estimates that over half of glaucoma patients do not follow their correct dosing schedules.
When medication fails to adequately reduce the pressure, laser trabeculoplasty often is performed. In laser trabeculoplasty, thermal energy from a laser is applied to a number of noncontiguous spots in the trabecular meshwork. It is believed that the laser energy stimulates the metabolism of the trabecular cells in some way, and changes the extracellular material in the trabecular meshwork. In approximately eighty percent of patients, aqueous outflow is enhanced and IOP decreases. However, the effect often is not long lasting and fifty percent of patients develop an elevated pressure within five years. The laser surgery is not usually repeatable. In addition, laser trabeculoplasty is not an effective treatment for primary open angle glaucoma in patients less than fifty years of age, nor is it effective for angle closure glaucoma and many secondary glaucomas.
If laser trabeculoplasty does not reduce the pressure enough, then filtering surgery is performed. With filtering surgery, a hole is made in the sclera and angle region. This hole allows the aqueous fluid to leave the eye through an alternate route.
The most commonly performed filtering procedure is a trabeculectomy. In a trabeculectomy, a posterior incision is made in the conjunctiva, the transparent tissue that covers the sclera. The conjunctiva is rolled forward, exposing the sclera at the limbus. A partial thickness scleral flap is made and dissected half-thickness into the cornea. The anterior chamber is entered beneath the scleral flap and a section of deep sclera and trabecular meshwork is excised. The scleral flap is loosely sewn back into place. The conjunctival incision is tightly closed. Post-operatively, the aqueous fluid passes through the hole, beneath the scleral flap and collects in an elevated space beneath the conjunctiva. The fluid then is either absorbed through blood vessels in the conjunctiva or traverses across the conjunctiva into the tear film.
Trabeculectomy is associated with many problems. Fibroblasts that are present in the episclera proliferate and migrate and can scar down the scleral flap. Failure from scarring may occur, particularly in children and young adults. Of eyes that have an initially successful trabeculectomy, eighty percent will fail from scarring within three to five years after surgery. To minimize fibrosis, surgeons now are applying antifibrotic agents such as mitomycin C (MMC) and 5-fluorouracil (5-FU) to the scleral flap at the time of surgery. The use of these agents has increased the success rate of trabeculectomy but also has increased the prevalence of hypotony. Hypotony is a problem that develops when aqueous flows out of the eye too fast. The eye pressure drops too low (usually less than 6.0 mmHg); the structure of the eye collapses and vision decreases.
Trabeculectomy creates a pathway for aqueous fluid to escape to the surface of the eye. At the same time, it creates a pathway for bacteria that normally live on the surface of the eye and eyelids to get into the eye. If this happens, an internal eye infection can occur called endophthalmitis. Endophthalmitis often leads to permanent and profound visual loss. Endophthalmitis can occur anytime after trabeculectomy. The risk increases with the thin blebs that develop after MMC and 5-FU. Another factor that contributes to infection is the placement of a bleb. Eyes that have trabeculectomy performed inferiorly have about five times the risk of eye infection than eyes that have a superior bleb. Therefore, initial trabeculectomy is performed superiorly under the eyelid, in either the nasal or temporal quadrant.
In addition to scarring, hypotony and infection, there are other complications of trabeculectomy. The bleb can tear and lead to profound hypotony. The bleb can be irritating and can disrupt the normal tear film, leading to blurred vision. Patients with blebs generally cannot wear contact lenses. All of the complications from trabeculectomy stem from the fact that fluid is being diverted from inside the eye to the external surface of the eye.
When trabeculectomy doesn""t successfully lower the eye pressure, the next surgical step often is an aqueous shunt device. An aqueous shunt device of the prior art is a silicone tube that is attached at one end to a plastic (polypropylene or other synthetic) plate. With an aqueous shunt device, an incision is made in the conjunctiva, exposing the sclera. The plastic plate is sewn to the surface of the eye posteriorly, usually over the equator. A full thickness hole is made into the eye at the limbus, usually with a needle. The tube is inserted into the eye through this hole. The external portion of the tube is covered with either donor sclera or pericardium. The conjunctiva is replaced and the incision is closed tightly. Many problems exist with the current technology of aqueous shunt devices including scarring, failure, hypotony, and infection.
Some prior art references for glaucoma management have been directed at Schlemm""s canal, but these have not involved the placement of long-term, indwelling stents. For example, U.S. Pat. No. 5,360,399 teaches the placement of a portion of a plastic or steel tube in Schlemm""s canal with injection of a viscous material through the tube to hydraulically dissect the trabecular meshwork. The tube is removed from the canal following injection. Furthermore, relative to that portion within Schlemm""s canal, the ""399 device has a larger diameter injection cuff element, which serves as an adapter for injection and irrigation. Therefore, this device is not adapted for permanent placement within Schlemm""s canal.
A need exists, then, for a more physiologic system to enhance the drainage of aqueous fluid through Schlemm""s canal. Enhancing aqueous flow directly into Schlemm""s canal would minimize scarring since the angle region is populated with a single line of nonproliferating trabecular cells. Enhancing aqueous flow directly into Schlemm""s canal would minimize hypotony since the canal is part of the normal outflow system and is biologically engineered to handle the normal volume of aqueous humor. Enhancing aqueous flow directly into Schlemm""s canal would eliminate complications such as endophthalmitis and leaks.
The present invention is directed to a novel stent and an associated surgical method for the surgical correction of glaucoma in which the stent is placed into Schlemm""s canal to expand the canal""s dimensions and maintain its patency. The present invention therefore facilitates the normal physiologic pathway for drainage of aqueous humor into and through Schlemm""s canal. The present invention is further directed to providing a permanent, indwelling stent within Schlemm""s canal for glaucoma management.